Recently, OBD (Onboard Diagnostics) systems that detect and display deterioration and breakdowns occurring in vehicle components are coming into general use with the main objective of preventing destruction of the environment by air pollution and also to comply with exhaust gas restrictions on vehicles with internal combustion engines. Besides detecting and monitoring problems (unexpected breakdowns) in devices for processing exhaust gas and informing the driver via a warning display when a problem occurs, OBD systems also store details of the breakdown or failure in a memory. The installation of simple OBD systems that sense broken wires and disconnection in electrical circuits for each sensor is currently required by law. However, ever tougher exhaust gas restrictions and advanced technology for reducing or cleaning exhaust gas will require installation of sophisticated OBD systems capable of detecting vehicle exhaust gas levels from various sensor information and reliably detecting breakdowns in their early stages.
Under these circumstances, efforts were made to develop OBD systems for example for diagnosing the presence and extent of variations in the air/fuel ratio in cylinders in vehicles with multi-cylinder internal combustion engines. However diagnosing variations in the air/fuel ratio in cylinders in multi-cylinder internal combustion engines required installing an air/fuel ratio detector part (air/fuel ratio sensor) in each cylinder. Installing an air/fuel detector part in each cylinder causes higher costs so a number of technologies were proposed for estimating the air/fuel ratio in each cylinder by using just one air/fuel ratio detector part. However, virtually none of the proposed technology of the conventional art provided the required diagnostic accuracy and was not at a level allowing mass production. This proposed technology was therefore incapable of estimating the air/fuel ratio in each cylinder and accurately diagnosing variations or the extent of variations in engine cylinders.
Here, the expression, “variations in air/fuel ratio in cylinders” indicates a state where there is a large deviation in the rich state or lean state of the air/fuel ratio for one or multiple cylinders relative to the average air/fuel ratio for all cylinders.
A diagnostic control apparatus for internal combustion engines and relating to OBD systems is disclosed in JP-A No. 2007-255230. This device operates fuel injection valves installed in cylinders of internal combustion engines with multiple cylinders by using feedback to control the rotation speed of the output shaft to a target value; and also contains a diagnostic part to diagnose whether a problem has occurred in any of the fuel injection-valves based on increased fluctuations in output shaft rotations.